1. Technical Field
The present invention relates generally to fuel control systems and, more particularly, to a method of controlling combustion parameters of an internal combustion engine in a motor vehicle capable of operating on more than one type of fuel.
2. Discussion
Environmental and energy independence concerns have stimulated the development of alternative transportation fuels, such as alcohol fuels, for use in automobiles. Alcohol fuels include methanol and ethanol. A flexible fueled vehicle capable of operating on gasoline, or alcohol fuel, or any mixture of the two fuels, is therefore in demand. Modifications to the engine are necessary when operating on different fuels because of the different characteristics of each fuel. For example, an engine operating on ethanol or E85 (a blend of 85% ethanol and 15% gasoline) requires approximately 1.4 times the amount of fuel relative to gasoline at stoichiometry due to a lower energy content of the ethanol.
Air/fuel ratio in internal combustion engine design is typically considered to be the ratio of mass flow rate of air to mass flow rate of fuel inducted by an internal combustion engine to achieve conversion of the fuel into completely oxidized products. The chemically correct ratio corresponding to complete oxidation of the products is called stoichiometric. If the air/fuel ratio is less than stoichiometric, an engine is said to be operating rich, i.e., too much fuel is being burned in proportion to the amount of air to achieve perfect combustion. Likewise, if the air/fuel ratio is greater than stoichiometric, an engine is said to be operating lean, i.e., too much air is being burned in proportion to the amount of fuel to achieve perfect combustion. Alcohol fuels have a lower air/fuel ratio than gasoline at stoichiometric, so that the engine must be compensated for in the rich direction as the percentage of alcohol in the fuel increases.
For example, U.S. Pat. No. 5,119,671, entitled "Method for Flexible Fuel Control", hereby expressly incorporated by reference, discloses a fuel composition sensor that measures the percent alcohol content of the fuel and relays that information to an engine controller or the like so that dependent variables such as spark timing and air/fuel ratio can be adjusted accordingly.
If a fuel composition sensor is used to directly measure the alcohol concentration in the fuel, it is mounted in the fuel line upstream of the injectors. The output of the sensor corresponds to a fuel composition multiplier that modifies the engine fueling to account for the fuel/air requirement differences of the fuels. Because there is a transport delay between the time the fuel is read by the sensor and the time the fuel reaches the injectors, the sensor reading does not correspond to the fuel at the injectors until after a period of time elapses.
Previously, there was no way to determine when the alcohol concentration was changing or any way to modify the output of the fuel composition sensor to match the actual fuel blending at the injectors. Therefore, the value read by the sensor was directly applied to the engine fueling. This causes fueling errors based on the difference between the fuel composition read by the sensor and the fuel composition being injected into the engine.
Therefore, it would be desirable to delay the application of the fuel composition multiplier that corresponds to the sensor reading until the fuel composition read by the sensor reaches the injectors. It would also be desirable to filter the sensor output to match the actual fuel delivery which may be affected by the fuel mixing in the fuel lines and the design of the fuel rail and injectors.